Chemical composition and relative hydrophobicity of microbial exopolymeric substances (EPS) isolated by anion exchange chromatography and their actinide-binding affinities
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Understanding the chemical and physical properties of microbially produced, actinide-carrying biopolymers is essential for the prediction of their relative binding affinities and particle-interaction propensities that are affecting the transport of actinide elements. In addition, they form the basis for their oceanographic applications, such as using POC/234Th ratios to determine organic carbon fluxes and for tracing upper ocean particle dynamics. Furthermore, environmental applications of bioflocculation and bioremediation can benefit from such improved understanding. Exopolymeric substances (EPS) in "attached" and "non-attached" forms, produced by two bacteria and one alga, were extracted and purified using improved procedures. These biopolymers produced by the two bacteria, which were previously shown to contain strong 234Th complexing ligands, were further fractionated by semi-preparative HPLC-anion exchange chromatography in order to relate Th(IV)-binding ability to chemical composition. Eluted fractions were further characterized at both semi-molecular and molecular levels. It was found that the EPS consisted of more than one biopolymer, with distinct monosaccharide and amino acid compositions, but very similar molecular weight (~25kDa). 234Th binding strength was correlated to their uronic acid to organic carbon ratios, suggesting that uronic acids are the actual binding agents of 234Th, or more likely, proxy compounds for the actual macromolecular 234Th binding ligands. These compounds were, however, not pure uronic acids, but rather amphiphilic glycoproteins or proteoglycans. Hydrophobic contact areas (HCA) of "attached" EPS, as measures of their relative hydrophobicity and amphiphilicity, were higher than those of "non-attached" EPS, and all were highly correlated to the ratios of the FTIR areas of amide-I bands from proteins to those of COC bands from carbohydrates. This suggests that the relative hydrophobicity of the carrier biopolymers, which can be responsible for gel formation, particle aggregation and bioflocculation, was mainly regulated by their relative protein to polysaccharide contents. 2011 Elsevier B.V.
